The BBC reported yesterday that there's "doubt" about the deportation of Abu Qatada, following his arrest on Tuesday and now his appeal to the European Court of Human Rights – which the home secretary Theresa May says is out of time. So: is she right? Is the appeal out of time? How has the Home Office got into this apparent mess? And what if any difference does this appeal make?

The European Court's judgment in Abu Qatada's case was dated January 17th 2012. Of that there's no doubt; and it's irrelevant whether the government or anyone else was given notice of the judgment before, or received it later.

On the face of it, the English texts are ambiguous: it would have been clearer had article 43.1 had said either that reference could be made within three months beginning on and including the date of judgment, or else within three months after that date. You might think the French text is more clearly helpful to the government, since it emphasises that counting begins from the date of judgment – but even that's not completely clear. Yes, you clearly count from January 17; but the legal issue is whether, when you then point to January 18 on the calendar, you mentally count that as day 1 (the clock having notionally started at day zero on the 17th), or day 2 ("judgment day" having been day one).

The first point to make about this ambiguity is that it illustrates the importance of precision in legal texts. People are too often tempted to think that legal exactitude in drafting amounts to mere verbiage. Not so. The second is that it's a bit surprising that the Convention is unclear – but not amazing. It's not easy to achieve complete clarity in texts negotiated internationally, as these texts were in Protocol 11 to the ECHR. Whatever the government achieves at Brighton, I expect plenty of argument about what any new Protocol means.

How, then, to resolve this ambiguity? The first principle legally speaking must be to interpret these provisions of Convention in the light of their object and purpose – in accordance with Article 31 of the Vienna Convention on the Law of Treaties. In my view the purpose of the provisions is to give a clear and finite period for "appeal", and their purpose is best served by reading them firstly, as allowing as clear a period as possible, and secondly, by reading them at the margin favourably to applicants. To me, that argues in favour of seeing applicants as having a full, clear three calendar months in which to apply, i.e. three months after the date of the judgment. Such a period would begin the day after judgment and would last three full months expiring at midnight (which midnight, by the way? Strasbourg midnight, presumably) at the end of Tuesday 17th April. The appeal would be in time, on this reckoning.

Adam Wagner was typically quick and right on Twitter yesterday to draw our attention to the relevant cases: Praha v Czech Republic, and Otto v Germany, neither of which is specifically about this three-month period but rather the six-month time limit for bringing your claim to the ECtHR in the first place. I read both cases as supporting Abu Qatada's reading, rather than the Home Secretary's.

In Otto, the Court said

the day on which the final domestic decision is pronounced is not counted in the six-month period referred to in Article 35 § 1 of the Convention. Time starts to run on the date following the date on which the final decision has been pronounced

and in Praha that

the six-month period begins to run on the day after the date on which the final domestic decision was pronounced

both of which support Abu Qatada's contention that the three-month period began on January 18 – the day after its judgment was pronounced – and so expired at the end of April 17.

the period of three months within which referral may be requested runs starts to run on the date of the delivery of the judgment, irrespective of whether the party concerned may have learned of it at a later stage.

That doesn't really help any further because as in the text of the Convention itself, it's not clear whether the count starts with "judgment day" as day zero or day one. I think the answer must be found in purposive interpretation, supported by Otto and Praha.

How could the Home Office get mixed up about this, then? Some commentators on the media last night, with the complacent superiority that I know comes naturally to us commentators, expressed disbelief that the government didn't simply "phone the court". Things are not as simple as the commentator's imagination might suggest, though. I've no doubt government lawyers were on the phone to Strasbourg more than once.

There may well have been crossed wires here between Foreign Office Lawyers – who deal with the European Court of Human Rights and who formally conduct litigation in it – and Home Office lawyers and civil servants. The prime concern in Whitehall's mind must have been how long the government itself had to appeal the Abu Qatada judgment, which after all went his way in a practical sense (although a key element of its legal reasoning suited the government, as Theresa May agreed in Parliament on Tuesday). It's not at all surprising that, if it were asked how long the government had to appeal (and what other purpose could the UK have had to ask?) Court staff should cautiously have cited April 16. If I'd been an FCO lawyer asked to advise when the UK needed to appeal by, I'd have said April 16, to be safe.

To be fair to government lawyers (of whom I've been one), these time issues can be tricky. It often amazes non-lawyers that there can be confusion about questions like this, but one of the surprising things you learn at law school is that it's not obvious how you calculate time. Indeed, a whole section of the massive and brilliant law encyclopedia Halsbury's Laws is devoted to the law of time.

In government you have the added difficulty that your ministerial clients seem obsessed with time, and the room it gives to delay decisions, in contexts where giving precise advice on time is difficult. I've advised on many EU law cases where the time-limit for a UK response depended in part on complex rules involving additional days allowed to government that varied according to how long the post was assumed to take between Luxembourg and the national capital. It frustrated me enormously that the only legal advice ministers seemed to be interested in was how I calculated the precise date, and that no one seemed anxious to listen to whether I thought we could win, or in getting on with deciding what if any arguments we'd make. A week or two would go by, then at the next meeting the only question would again be: "Till when do we have?"

What I learned was that, in order to focus minds, I had to be precise, cautious and unambiguous about time limits. The worst thing that could happen would be to give a date that turned out to be too late. I also learned in government that it's best to avoid complexity in your answers, if possible: it's generally unwanted, and often interpreted as proof that you're a typical civil service ditherer, whose view can be ignored. That's why, as I've already said, if I'd been advising ministers in a meeting in a context where we ourselves were considering appealing, I'd have given Monday April 16 as the final day for appealing. No other answer would have been so clear and safe, or have commanded confidence. I'd have kept the hidden complexity to myself unless asked for written advice, or unless asked specifically about how long the other side might have to appeal.

What was really needed here was good, old-fashioned lawyers' advice. Regardless of when the time-limit technically ran out, the government would have been well advised to wait until at least midnight at the end of 17 April before treating the judgment as final. Theresa May's statement should have taken place after Prime Minister's Questions yesterday, rather than on Tuesday. But often in modern government, advice in that broad sense seems unwelcome: if they offer it, lawyers can be told sharply that all ministers want is a simple technical answer to a simple technical question such as "how long do we have to appeal". In this case, especially if as I suspect a government appeal was a live issue, then the only good answer to give was Monday April 16. I wonder whether this impatient, advice-averse culture and the "garbage in, garbage out" sort of lawyering it can spawn might be partly to blame for the apparent muck-up here.

Finally, some people will wonder why on earth Abu Qatada's lawyers would have waited till the last moment – and possibly too late – to enter their appeal. Well, first, I've already said I think their view of the time-limit is correct. They're in time, which is all that matters. But there may be a practical explanation for why the application came so late.

It's worth remembering that Abu Qatada in effect won on January 17: the European Court of Human Rights decided he couldn't be deported because of the possible use in Jordan of evidence obtained by torture. It's not obvious why he'd want to appeal that ruling, or whether they have serious grounds to do so. But what his lawyers certainly want is to find some procedural means of preventing deportation now, if they can. There's a possibility of a final appeal to the Special Immigration Appeals Commission against deportation – in which case, deportation itself will be suspended pending the outcome. But given that Siac and the higher UK appeal courts have already ruled his deportation lawful, even before the most recent agreement with Jordan, Abu Qatada must know those proceedings offer him little hope, and may not be drawn out all that long.

He must have hoped that the government – which after all lost on January 17 and has much better grounds on which to contest it – would appeal the ECtHR's judgment, therefore keeping proceedings in that court alive and enabling him if need be, once deportation action was revived, to ask Strasbourg again for interim measures to temporarily block it. Once it became clear from Theresa May's Commons statement that the government was not appealing the judgment, but believed it could effect a deportation compliant with it, Abu Qatada's lawyers finally realised the only way the existing interim measures could be kept in existence or further interim measures applied for would be their own last-minute appeal, however odd that might seem. No doubt the papers had already been prepared in advance – his lawyers are certainly a capable lot – and so were ready to be sent to the court on the day or his arrest, after a few final hours work.

Oddly, what was forgotten in yesterday's media flurry about "confusion" and "doubt" was that this appeal makes little real difference, in the scheme of things. Theresa May told the Commons on Tuesday that she expected deportation could take "many months" because of appeals – this just proves her right. And deciding not to arrest Qatada till Wednesday would not actually have prevented an appeal being made on Tuesday.

I don't think there's any question of Tuesday's arrest being unlawful, by the way. What the interim measures prevent is deportation itself, rather than arrest or detention with a view to deportation.

I expect Abu Qatada to land in Jordan eventually; but not before Julian Assange lands in Sweden.

This post by Carl Gardner first appeared on the Head of Legal blog and was then reproduced with permission by the UK Human Rights Blog. The original version of this article incorrectly referred to Monday April 17, instead of Monday April 16. This has now been corrected.